(1) Field of the Invention
The present invention relates to a foamed rubber extruded product formed of a vulcanizate of a rubber blend comprising a vulcanizable ethylene-propylene-type rubber (hereinafter referred to as "EPDM") as a starting rubber and a pyrolytic foaming agent.
(2) Description of Related Art
The present invention is described by taking a weather strip as an example. However, the present invention can also be applied to weather strip rubbers such as a window frame rubber and a glass Sun, automobile interior and exterior rubber products such as an opening trim and a head lamp seal, a hose and a rubber mandrel.
A door weather strip shown in FIGS. 1 and 2 has a U-shaped grip 14, which is formed of a solid rubber, having an insert 12 embedded therein, and a hollow seal 16 and a lip seal 18 which are formed of a sponge rubber.
Usually, the above-mentioned U-shaped grip 14 is formed by extrusion-molding a vulcanizate (solid rubber) of an unformed EPDM rubber blend. EPDM is excellent in weatherability and in ozone resistance and good in low-temperature properties so that it can easily satisfy the requirements of a weather strip.
In order to meet the recent requirements of a weather strip, such as low cost and light weight, it is considered that a U-shaped grip is formed by extrusion-molding a foamed rubber blend such as proposed in Japanese Patent Laid-Open No. 87,411/1997. When this foamed rubber blend is used, a foamed rubber extruded product having a specific gravity of from approximately 0.9 to 1.0 and a surface roughness (RZD) of 14 micrometers or less, a so-called finely foamed rubber extruded product, can easily be obtained. When the specific gravity is less than 0.9, a predetermined hardness and a predetermined rigidity are hardly provided, and the product cannot be a substitute for the solid rubber. RZD refers to the roughness indicated by the mean value of the heights of ten points of irregularities. Meanwhile, when it exceeds 1.0, the requirement for lowering weight is scarcely satisfied. Further, when the surface roughness (RZD) exceeds 14 micrometers, a poor appearance (poor surface roughness) is provided.
However, in the case of the foamed rubber blend containing the above-mentioned pyrolytic foaming agent, it has been found that the extrusion at a high rate brings the following problems. The high rate refers to approximately 8 m/min or more. For example, the usual extrusion rate is approximately 5 m/min (refer to [0033] in the above-mentioned Japanese Patent Laid-Open No. 87,411/1997).
(i) The surface roughness is easily increased, and a product having a specific gravity of 1.0 or less is hardly obtained (refer to Comparative Example 2').
According to the present inventors' study, this is presumably because an extrusion head pressure is relatively high (refer to FIG. 3).
(ii) When the weather strip having the finely foamed rubber portion (U-shaped grip 14) and the sponge rubber portions (hollow seal 16 and lip seal 18) as shown in FIGS. 1 and 2 is formed by the extrusion, material is returned to the initial stage of the extrusion, and the sponge rubber portion lacks a dimensional stability, so that unsatisfactory extrusion tends to occur.
The returning of the material occurs presumably because, as shown in FIG. 3, a peak value P1 of an initial extrusion head pressure of the finely foamed rubber (Comparative Example 2') is higher than a peak value P2 of an ordinary solid rubber (Comparative Example 10). [Refer to FIG. 3.]
Especially when the U-shaped grip 14 is extruded from the finely foamed rubber portion at the same time the hollow seal 16 and the lip seal 18 are extruded from the sponge rubber portions, as shown in FIGS. 1 and 2, the dimensional stability of the sponge rubber portions becomes lower than in the simultaneous extrusion from the solid rubber portion. This is because by the returning of the material of the finely foamed rubber portion, the finely foamed rubber enters into the sponge rubber portion and is extruded thereinto, with the result that a portion which is to be originally formed with the sponge rubber portion is formed with the finely foamed rubber portion instead.